Quikrete Calculator
Quikrete sells concrete in 80, 60, and 40-lb bags — and picking the right count starts with volume, not guesswork. A Quikrete Calculator converts any slab, footing, or post-hole project into cubic feet and yards, then tells you exactly how many bags of each size to buy, with a waste allowance baked in.
This guide covers bag yields by size, the volume math for rectangles and round holes, how to use the calculator, and two fully worked projects. You will also learn which bag size to choose, when to switch to ready-mix, and the mistakes that leave DIYers short mid-pour.
Bag Yields: What Each Size Makes
Every Quikrete bag states its yield — the volume of mixed concrete it produces. For standard concrete mix: an 80-lb bag ≈ 0.60 cu ft, a 60-lb bag ≈ 0.45 cu ft, and a 40-lb bag ≈ 0.30 cu ft. Fast-setting mixes yield slightly less per pound (80-lb ≈ 0.50 cu ft) because of their different formulation.
The universal conversion: 45 eighty-pound bags = 1 cubic yard (27 cu ft ÷ 0.60). Handy equivalents: 60 sixty-pound bags or 90 forty-pound bags per yard. These numbers are the Rosetta Stone of concrete estimating — memorize 45/60/90 and you can sanity-check any project in your head.
Volume Math for Every Shape
Slabs and footings (rectangular): length × width × thickness, with thickness converted from inches to feet (÷ 12). A 12×10 ft slab at 4": 12 × 10 × (4/12) = 40 cu ft.
Post holes (cylindrical): π × r² × depth, all in feet, times the number of holes. A 12"-diameter, 36"-deep hole: π × 0.5² × 3 = 2.36 cu ft per hole. The classic blunder is using diameter instead of radius — that quadruples your estimate.
Bags: volume ÷ yield per bag, × (1 + waste), rounded up. A 10% allowance handles spillage, uneven subgrade, and form leakage on typical work.
Which Bag Size Should You Buy?
80-lb bags are cheapest per cubic foot but brutal to handle — 45+ bags is a serious workout and multiple vehicle trips. Choose them for big slabs where cost dominates. 60-lb bags are the DIY sweet spot: manageable weight, widely stocked, still economical. 40-lb bags cost more per yard but suit small repairs, single post holes, and anyone working solo without a truck.
Rule of thumb: under ~10 bags, buy whatever size you can comfortably carry; over ~45 bags (1 yard), price a ready-mix truck — it is usually cheaper per yard, far faster, and eliminates cold joints. The calculator's yard figure tells you exactly where you stand.
Concrete vs. Mortar vs. Grout: Buying the Right Bag
The Quikrete aisle is a wall of similar-looking bags, and grabbing the wrong one is an expensive mistake. Concrete mix contains cement, sand, and gravel — the aggregate gives it structural strength for slabs, footings, and post holes. Mortar mix contains cement, sand, and lime but no gravel — it is a sticky bedding compound for laying block, brick, and stone, with a fraction of concrete's compressive strength. Grout (sold as such, or site-mixed) is fluid concrete for filling narrow spaces like block cores and tile joints.
The failure modes of confusion are specific and sad: a patio poured with mortar mix will dust, crack, and crumble under foot traffic within a season — it was never meant to be a wearing surface. Block laid with concrete mix instead of mortar produces fat, weak joints full of gravel that will not tool properly. And fast-setting variants (concrete or post mix) trade working time for speed — wonderful for fence posts, terrible for a slab you need 45 minutes to screed. Read the bag's stated purpose, match it to your project, and when in doubt ask: the three products are chemically different tools, not interchangeable powders.
How to Use This Quikrete Calculator
Pick your project shape — slab/footing or round post holes — and enter the dimensions. Choose a waste allowance (10% is the safe default) and your mix type (standard or fast-setting, which changes the yields). Press Calculate.
Results show volume in cubic feet and yards plus bag counts for all three bag sizes (waste included, rounded up), and the total haul weight for the 80-lb option so you can plan transport.
Worked Example 1: A 12×10 Shed Slab at 4 Inches
A 12 ft × 10 ft shed foundation, 4 inches thick, standard mix, 10% waste.
Step 1 — Volume. 4 ÷ 12 = 0.333 ft. 12 × 10 × 0.333 = 40.0 cu ft = 1.48 cu yd.
Step 2 — 80-lb bags. 40 ÷ 0.60 = 66.7 → × 1.10 = 73.3 → 74 bags.
Step 3 — Other sizes. 60-lb: 40 ÷ 0.45 = 88.9 → × 1.10 → 98 bags. 40-lb: 40 ÷ 0.30 = 133.3 → × 1.10 → 147 bags.
Step 4 — Decision. 74 × 80 = 5,920 lbs and 1.48 yards — past the 1-yard crossover. Order a ready-mix truck (order 1.75 yards to be safe) instead of hand-mixing 74 bags.
Worked Example 2: Six Fence Post Holes
6 holes, each 12" diameter × 36" deep, fast-setting mix, 10% waste.
Step 1 — Volume per hole. Radius 6" = 0.5 ft; depth 36" = 3 ft. π × 0.5² × 3 = 2.356 cu ft.
Step 2 — Total volume. 2.356 × 6 = 14.14 cu ft = 0.52 cu yd.
Step 3 — Bags (fast-setting yields). 80-lb: 14.14 ÷ 0.50 = 28.3 → × 1.10 → 32 bags. 60-lb: 14.14 ÷ 0.375 = 37.7 → × 1.10 → 42 bags. 40-lb: 14.14 ÷ 0.25 = 56.6 → × 1.10 → 63 bags.
Step 4 — Practical pick. 42 sixty-pound bags of fast-setting mix — one bag per hole per... actually ~7 bags per hole; set each post with dry mix + water in the hole per the bag instructions. Well under truck territory: bags win here.
Footings, Piers, and Other Shapes
Slabs and post holes cover most DIY work, but the same volume math handles everything else. Continuous footings (the concrete strip under a wall or foundation) are just long, narrow slabs: a 30-foot footing, 16 inches wide and 12 inches deep, is 30 × (16/12) × (12/12) = 40 cu ft — about 67 eighty-lb bags. Square piers (for decks) are length × width × depth per pier, times the pier count: eight 12×12×36-inch piers = 8 × 1 × 1 × 3 = 24 cu ft ≈ 40 bags.
Steps need the triangle treatment: each step is roughly a triangular prism — (tread × riser ÷ 2) × width — times the number of steps, plus the landing slab. Five steps with 11-inch treads, 7-inch risers, and 4-foot width: 5 × (11/12 × 7/12 ÷ 2) × 4 ≈ 5.3 cu ft, plus landing. Irregular shapes — curved patios, for instance — get the grid method: sketch on graph paper, count full and partial squares for area, multiply by thickness. The calculator's slab mode handles any shape once you have its area; the skill is in measuring the area honestly, including the extra depth where the subgrade dips.
Common Estimating Mistakes
Mixing inches and feet is the #1 error — always convert thickness to feet before multiplying. Forgetting waste leaves you short; subgrades are never perfectly level. Diameter-vs-radius on round holes quadruples the estimate. Ignoring haul weight strands people at the store: 74 bags is three tons — plan the truck or delivery. And buying fast-setting for big slabs is a trap: it sets before you can screed a large area. Fast-setting is for posts and small patches only.
Ordering Ready-Mix: How It Works
Crossing the 1-yard threshold into ready-mix territory introduces a new process worth understanding. You order by cubic yards (round up — order 2 yards for a 1.7-yard pour), specify the mix design (strength in psi, aggregate size, and slump — the stiffness measure; a 4-inch slump is standard for slabs), and schedule a delivery window. The truck carries up to ~10 yards; most companies charge a short-load fee below 3–5 yards, so a 1.5-yard order costs disproportionately more per yard than a 6-yard one — get quotes, and consider combining projects with a neighbor.
On delivery day, preparation decides everything: forms staked and level, subgrade compacted, help on site (you need 2–3 people minimum to screed and finish), and truck access confirmed — the truck needs to get within chute range (~15–20 feet) of the pour, or you will pay for a pump or wheelbarrow brigade. Once the truck arrives, the clock runs: you typically get 60–90 minutes of discharge time before standby charges start. Have everything ready before the truck, not during. Done right, a ready-mix pour that would have been 200 bags of misery becomes a single smooth morning — which is exactly why the calculator flags the crossover.
Tips for Buying and Pouring
- Memorize 45/60/90. Bags per yard for 80/60/40-lb sizes — instant sanity checks anywhere.
- Add 1–2 spare bags. Unopened bags return; short pours create cold joints.
- Convert inches to feet first. Thickness ÷ 12 before multiplying — every time.
- Price ready-mix past 1 yard. ~45 eighty-pound bags is the crossover point.
- Match mix to job. Fast-setting for posts and patches; standard for slabs and footings.
- Plan the haul. Check total weight before promising your sedan can carry it.
- Keep bags dry. Store off the ground under cover; moisture hardens bagged mix.
- Pour continuously. Stage all bags and water before the first mix — no mid-pour store runs.
- Buy all bags from the same lot when possible. Same-batch bags have identical color and set characteristics — important for visible work like countertops or exposed slabs.
- Stage bags near the pour, not the truck. Every bag gets carried twice if you unload far away — plan the carry path before the delivery arrives.
- Keep a few bags dry for touch-ups. Edge repairs and small patches always come up — dry-stored bags from the same project match perfectly.
Storing and Handling Bags Safely
Bagged concrete is heavy, caustic, and perishable — treat it with respect. An 80-lb bag is at the edge of safe solo lifting; use proper form (lift with the legs, keep the load close), team-lift when stacking high, and never throw bags — ruptured bags waste material and create caustic dust. Portland cement is caustic: wet mix burns skin with prolonged contact, and the dust irritates eyes and lungs. Wear gloves, eye protection, and a dust mask when handling dry bags; wash off splashes promptly rather than letting them dry on skin.
Storage decides whether leftover bags survive to the next project. Moisture is the enemy: a bag that absorbs humidity hardens into a useless lump within weeks. Store bags indoors or under a waterproof tarp, off the ground on pallets — never directly on a concrete floor, which wicks moisture upward. Rotate stock (first in, first out), and squeeze-test old bags before trusting them: a bag with hard lumps has partially hydrated and will not reach rated strength. When in doubt, use questionable bags for non-structural work (setting a mailbox post) and fresh bags for anything load-bearing. A $7 bag is never worth a failed footing.
Frequently Asked Questions
1. How many bags of Quikrete in a cubic yard?
45 eighty-lb bags, 60 sixty-lb bags, or 90 forty-lb bags of standard mix per cubic yard, before waste.
2. What does an 80-lb bag of Quikrete yield?
About 0.60 cubic feet of mixed concrete (0.50 for fast-setting). That covers 1.8 sq ft at 4" thick.
3. 60-lb or 80-lb bags — which should I buy?
80-lb is cheaper per yard; 60-lb is easier to handle. For DIY projects under ~30 bags, 60-lb bags save your back for a small premium.
4. How do I calculate for a round post hole?
π × radius² × depth (all in feet) × number of holes. A 12"×36" hole is ~2.36 cu ft — about 4 eighty-lb bags of standard mix.
5. When should I order ready-mix instead?
Above ~1 cubic yard (45 eighty-lb bags). Trucks are cheaper per yard, faster, and give a monolithic pour with no cold joints.
6. What is the waste allowance for?
Spillage, form leakage, uneven subgrade, and partial bags. 10% is standard; use 15% on rough hand-dug work.
7. Can I use fast-setting mix for a slab?
Not recommended — it stiffens too fast to screed and finish a large area properly. Use standard mix for slabs.
8. How much water per bag?
Roughly 3 quarts per 80-lb bag, but follow the bag directions and add gradually — soupy mix is weak mix.
9. How thick should my slab be?
4" for patios, walkways, and sheds; 5–6" for driveways; thicker for structural footings per your plans.
10. Do I need to add gravel under the slab?
Yes — 4" of compacted gravel base prevents settling, improves drainage, and stops the #1 cause of cracked DIY slabs.
11. How long until I can walk on it?
About 24 hours for foot traffic; wait 7 days for vehicles and 28 days for full design strength.
12. Why did my estimate come up short?
Usually unconverted inches, forgotten waste, or a subgrade that was deeper than measured. Re-measure depth in several spots.
13. Can I mix bags from different brands?
For non-structural work it is acceptable, but strengths and set times vary — stick to one product per pour for consistency.
14. How should I store leftover bags?
Indoors or under a tarp, off the concrete floor, sealed. A bag that has absorbed moisture is ruined — when in doubt, throw it out.
15. Is the calculator accurate for footings?
Yes — footings use the same length × width × depth math. Just be sure trench depth is measured after the loose soil is cleaned out.
Every concrete project is the same three steps: measure honestly, convert to volume, divide by yield. Memorize the 45/60/90 rule, respect the waste allowance, buy the right product for the job, and call the ready-mix truck when the yardage says so. Concrete forgives almost nothing — but it richly rewards the estimator who did the math first. Measure twice, pour once — the oldest rule in concrete, and still the best one, because every bag you did not have to buy twice is money kept. Plan it right.
CONCLUSION
A Quikrete Calculator replaces guesswork with a shopping list: exact volume in cubic feet and yards, bag counts for 80, 60, and 40-lb sizes with waste included, and the total weight you will haul. The shed slab example drew the line at 74 bags — truck territory; the fence posts landed at 42 sixty-pound bags — a perfect DIY Saturday.
Memorize the 45/60/90 rule, convert your inches, add the waste, round up, and keep a spare bag or two. Concrete is unforgiving of shortages and generous to the prepared — measure twice, pour once.